1. Field of the Invention
The present invention relates to a switching power source that generates a direct-current (DC) voltage.
2. Description of the Related Art
With an increase in demand for power saving electronic devices that reduces power consumption in various fields in recent years, more power saving is also required of power sources which supply electrical power to the electronic devices. The schematic configuration diagram of a switching power source which is one example of power sources for electronic devices is illustrated in FIG. 15. In FIG. 15, an alternating-current (AC) voltage input from a commercial alternating current power source 100 is input into a transformer 104 via a rectification unit 140, and a switching element 108 such as a field effect transistor (FET) performs switching operation at a predetermined frequency, based on a signal sent out from a control circuit 144, thereby a primary side of the transformer 104 is driven. Then, the DC voltage V is generated by smoothing a voltage generated on a secondary side of the transformer 104 by a smoothing unit 141.
A switching power source that generates the desired DC voltage in this manner by driving the switching element 108 at the predetermined frequency is widely used. Among such switching power sources, there are some that improve operation efficiency by decreasing a number of switching times of the switching element 108 (by decreasing a switching frequency), for example, during power-saving operation (sometimes referred to as during light-load running) in which an electronic device is not operating.
Most of losses of the switching power source during the light-load running are losses of the switching operation, and in order to reduce the losses, energy for one switching operation is increased by prolonging a time during which the switching element 108 is ON (sometimes referred to as ON-period). Thus, it is being attempted to decrease the number of switching times per unit time by extending an idle (pause) period.
However, as the idle period is extended, the switching frequency is decreased much more, and there arises a possibility that a sound produced by the transformer associated with the switching operation comes into an audible range. Further, since this sound contains harmonic wave components, it becomes a sound disagreeable to the human ear.
Hereinbelow, the reason that the switching frequency results in a sound containing harmonic waves will be described. When the switching frequency becomes several kHz or less, the idle period of the switching element becomes longer. As a result, the driving current waveform of the transformer takes a delta-function waveform as illustrated in FIG. 16. In this case, FIGS. 16A and 16B illustrate transformer driving current waveforms, and driving pulse waveforms, when the switching element is driven by a 1-wave driving pulse with a cycle of 1 msec, and ON-period of 5 μsec. FIG. 16A illustrates the driving current waveforms of the transformer, in which the vertical axis indicates transformer driving current (A) and horizontal axis indicates time (sec). FIG. 16B illustrates the driving pulse waveforms, in which the vertical axis indicates drive voltage (V) and the horizontal axis indicates time (sec). The results of the frequency analysis (Fast Fourier Transform analysis (sometimes referred to as FFT analysis)) performed on such the transformer driving current waveforms are illustrated in FIG. 17.
In FIG. 17, the vertical axis indicates transformer driving current (mA), and the horizontal axis indicates frequency (Hz). As illustrated in FIG. 17, the transformer driving current, with the switching frequency set as a fundamental wave, has harmonic wave components with a multiplied frequency. The transformer driving current takes a current waveform having energy driven by the harmonic wave components. Further, the transformer of the switching power source is also driven at a predetermined resonance frequency as the switching operation. Mechanical resonance frequency of the transformer is also dependent on a core shape of the transformer, and generally, has a peak of the resonance frequency within a frequency band of several kHz to several tens of kHz.
For example, as illustrated in FIG. 16, one wave driving pulse is applied to the switching element, and the device is driven with use of a transformer having a resonance level at which mechanical resonance frequency is close to a frequency band having a peak approximately at 18 kHz. Sound pressure of the beat sound produced from the transformer at this time is illustrated in FIG. 18.
In FIG. 18, the vertical axis indicates sound pressure (dB) of the beat sound of the transformer, and the horizontal axis indicates frequency (Hz), wherein sound pressure contains harmonic waves where the envelope exhibits mechanical resonance frequency characteristics of the transformer, with the switching frequency set as a fundamental wave. In other words, when the switching frequency and the mechanical resonance frequency of the transformer overlap each other, a disagreeable sound to the ear is generated, which comes into the audible range as the beat sound from the transformer.
As one of methods for reducing the production of such the beat sound from the transformer, a method for reducing the beat sound by suppressing a magnetic field change rate of the transformer is known. Conventionally, to suppress the magnetic field change rate of the transformer, there has been employed a method of using the transformer with a core material having a large sectional area, or reducing a switching current of the transformer per one operation by shortening ON-period of the switching element.
Further, as a method for reducing the production of the beat sound of the transformer by devising the transformer driving current waveforms, a soft start circuit is provided in a switching power source, and duty ratio is gradually changed at the time of ramp-up and ramp-down of a voltage of both ends of the capacitor during startup operation. If the current wave is formed such that a size of the transformer driving current waveform is gradually increased, or gradually decreased, magnetic flux change of the transformer can be decreased, and as a result, the production of the beat sound can be reduced. Such conventional schemes are discussed in, for example, Japanese Patent No. 3567355 and Japanese Patent No. 3665984.
However, if a core material having a large sectional area is used for the transformer, it becomes difficult to reduce size of the power source. Further, in a method of shortening ON-period of the switching element, although the production of the beat sound of the transformer is reduced by decreasing the magnetic flux change of the transformer, a number of switching times per unit time will be increased, and switching losses will be increased.
Further, in the case of the method of gradually increasing, or gradually decreasing the amplitude of the transformer driving current waveform by the soft start, during the light-load running, when power consumption is to be more reduced, application of the soft start becomes difficult, since energy supplied to a secondary side load becomes less. This is because, if energy supplied to the secondary side becomes less during the light-load running, it becomes difficult to gradually increase or decrease the amplitude of the current waveform by the soft start circuit.
Furthermore, in the conventional method, switching must be performed more times by reducing energy supplied in one switching operation, or the capacitance of a capacitor on the secondary side must be increased to be severalfold without changing the energy supplied in one switching operation. The former method increases switching losses, greatly lowering efficiency. The latter method increases product costs. In other words, in the switching power source, the switching losses are desirably reduced by decreasing the number of switching time. However, in this case, energy per wave which is applied to the transformer by a driving pulse is increased, and a greater sound is generated.